Light-receiving circuits for receiving light signals have heretofore used configurations as shown in FIGS. 4 and 5, in each of which the configuration comprises a photosensitive device 1, a DC amplifier 2, a comparator 3, a peak detector 4, an average value output portion 5 that is formed by an integrator circuit, resistors R1-R4, and a capacitor C1. The comparator 3 has a first input terminal 6, a second input terminal 7, and an output terminal 8.
In the configuration shown in FIG. 4, a signal that is converted into an electrical signal by the photosensitive device 1 is amplified by the DC amplifier 2 without losing its DC component. Then, the signal is supplied to the first input terminal 6 of the comparator 3. The output from the amplifier 2 is also applied to the peak detector 4, which delivers an output whose magnitude, or threshold value TH indicated by the broken line in FIG. 6(A), is half, for example, of the peak value SG indicated by the solid line in FIG. 6(A). This output is fed to the second input terminal 7 of the comparator 3. Thus, a rectangular waveform appears at the output terminal 8 of the comparator 3, and this waveform assumes a high level during the periods in which the signal SG is in excess of the threshold value TH, as shown in FIG. 6(B).
In the configuration shown in FIG. 5, the output from the DC amplifier 2 is applied to the first input terminal 6 of the comparator 3 in the same way as in the configuration shown in FIG. 4. Under this condition, however, the average value output portion 5 delivers an output, which is derived by integrating the signal SG indicated by the solid line and constitutes the threshold value TH indicated by the broken line in FIG. 7(A), because of the integrating function of the capacitor C1. As a result, a rectangular waveform appears at the output terminal 8 of the comparator 3, and this waveform is at a high level during the periods in which the signal SG is larger than the threshold value TH, as shown in FIG. 7(B).
Heretofore, shaped electrical signals have been obtained in the manner as described above. Also as mentioned previously, it is desired that the output signal from the photosensitive device 1 be amplified without losing its DC component, thus necessitating the use of the expensive DC amplifier 2.